1. Field of the Invention
The present invention relates to a rotation position detecting device that detects a reference position of a rotating object by means of a rotation signal generated in synchronism with rotation of the rotating object.
2. Description of the Related Art
U.S. Pat. No. 5,264,844 (or its one of basic Japanese patent applications JP-A-Hei 5-66105) discloses a rotation position detecting device that counts the number of clock signals to calculate the cycle time of the pulses of a rotation angle signal. Such a rotation detecting device includes an up-down counter and a f/K frequency dividing circuit, in which a ⅓ frequency dividing circuit and a ½ frequency dividing circuit are combined to provide a down-clock. The above-stated rotation position detecting device measures the cycle time of pulses by counting the number of clock signals, from which the down-clock counts down. The reference position can be detected when a borrow signal appears. U.S. Pat. No. 5,264,844 (or its another basic Japanese patent application JP-A-Hei 5-71909) also discloses a rotation position detecting device as shown in FIGS. 10, 11, 12A and 12B. In this rotation position detecting device 1, a frequency dividing circuit 4 divides an angle signal NE that is sent from a rotation sensor 2 via a wave-shaping circuit 3 into a half to provide a pair of up-down command signals SUD1 and SUD2. A pair of up-down counters 5 and 6 counts up or counts down alternately according to the up-down command signals SUD1 and SUD2 at each cycle time of the angle signal NE.
The direction of counting up or down by the up-down counter 5 is opposite to the direction of counting up or down by the up-down counter 6. Each up-down counter counts up according to an up-clock signal CLKU that is provided by a clock circuit 7 and counts down according to a down-clock signal CLKD that is provided by a 1/K frequency dividing circuit 8. The clock signals applied to the up-down counters 5, 6 are controlled by selectors 9, 10. When edge detecting circuits 11, 12 respectively detect an up-edge of the up-down command signals SUD1, SUD2, the counted values of the up-down counters 5, 6 are reset to 0.
When the rotation sensor 2 detects a non-tooth portion (in an uneven cycle time), the count number of one of the up-down counters 5, 6 becomes 0, so that a borrow signal or detection signal BO is outputted. Because there are two states of the down clock signals CLKD when the up-down command signals changes, the timing of transition from the up-count to the down-count may shift, so that the detection signal BO may not be provided at a right timing, as shown in FIGS. 12A–12E and 13A–13D.
Because the down clock signal CLKD is formed by dividing the up clock signal CLKU into 1/K according to the number of non-toothed portions, the frequency of the down clock signal CLKD becomes lower than the up clock signal CLKU. Therefore, a half-cycle shift of the down clock signal CLKD becomes K times as long as a half cycle of the up clock signal CLKU. If the shift of the detection signal BO becomes larger, the frequency of the clock signal has to be increased, resulting in increasing power of the clock circuit.